The present invention relates to an improvement in an opposed magnet-type magnetic circuit assembly with permanent magnets or, more particularly, relates to an improvement in an opposed magnet-type magnetic circuit assembly with permanent magnets having usefulness for generating a magnetic field of high uniformity required in various kinds of precision magnetic instruments such as NMR, ESR and MRI instruments in which uniformity of the magnetic field within a relatively large space is essential.
Several types of magnetic circuit assemblies with permanent magnets have been heretofore proposed and now under practical applications, for example, in permanent magnet MRI instruments with an object to obtain a highly uniform magnetic field within a large space. One of the most conventional types is the so-called dipole-ring magnetic circuit assembly illustrated in FIG. 1 by a perspective view. The main part of the magnetic circuit assembly is a tubular assembly 1 consisting of a plural number of segment magnets 1A to 1H arranged in a tubular arrangement, each having a direction of magnetization indicated on the end surfaces of the segment magnets 1A to 1H by the respective arrows, the directions rotating twice starting with the segment magnets 1A and ending with 1H in a counterclockwise circulation on the figure. The name of the "dipole-ring" magnetic circuit is given in view of the appearance of the magnetic charges of N and S on the inward surface of the tubular assembly 1 in the revolution of the magnetization direction. As a synthesis of the magnetic fields generated by the respective segment magnets 1A to 1H, a magnetic field indicated by the arrow H is generated in the space surrounded by the tubular arrangement of the segment magnets 1A to 1H. The uniformity of the magnetic field H can be high enough provided that the tubular assembly 1 of the segment magnets 1A to 1H is sufficiently long. Adjustment of the magnetic field H can be performed by the displacement of the respective segment magnets 1A to 1H in the radial directions or modification of the inclination thereof in the axial directions.
The magnetic circuit assembly of this type is described, for example, by K. Halbach in Nuclear Instruments and Methods, volume 169 (1980), page 1 and MRI instruments utilizing a magnetic circuit assembly of this type are disclosed in U.S. Pat. No. 4,580,098 and Japanese Patent Kokai 62-104011.
Since no yokes are required outside of the tubular magnet assembly 1 to serve as a pathway of the magnetic flux, the dipolering magnetic circuit assembly has advantages, as compared with magnetic circuit assemblies of other types using yokes, that the overall weight of the magnetic circuit assembly can be relatively small and the upper limit of the magnetic field obtained therein can be high. The advantage of the smaller overall weight, however, is exhibited only when the magnetic field to be generated therein is considerably high because, when the magnetic field required is only about 3000 Oe or lower, the total weight of the segment magnets can not always be smaller than in the magnetic circuit assemblies of other types. This is the reason for the relatively small number of cases where magnetic circuit assemblies of this type are used in a practical application.
Another type of the magnetic circuit assembly widely used in the above mentioned applications is an opposed-magnet magnetic circuit assembly illustrated in FIG. 2A by an axial cross sectional view and in FIG. 2B by a horizontal cross sectional view as cut and viewed in the direction indicated by the arrows IIB--IIB in FIG. 2A. Magnetic circuit assemblies of this type are disclosed, for example, in WO84/00611 (PCT/US 83/01175), Japanese Utility Model Publications 2-44483, 2-44484, 2-44485 and 2-44486, Eizo Joho (Imaging Information), volume 15 (1983), page 379, Byotai Seiri (Disease Physiology), volume 4 (1985), page 91 and elsewhere. Almost all of the permanent-magnet MRI instruments currently running are built with a magnetic circuit assembly of this type.
Following is a brief description of the general structure of the opposed-magnet magnetic circuit assembly.
An upper magnet 14A and lower magnet 14B are respectively mounted on the lower surface of the upper back yoke 10A and on the upper surface of the lower back yoke 10B in a coaxial disposition directing the S-pole and N-pole, respectively, toward the magnet gap to generate a magnetic field therein. A field-adjustment plate 16A or 16B made from a magnetic material is mounted on the surface of the magnet 14A or 14B, respectively, facing the gap space with an object to improve the uniformity of the magnetic field generated in the gap space between the magnets 14A and 14B. The field-adjustment plates 16A and 16B each have an axially symmetrical configuration or in the form of a disk in this case, in most cases, with modifications of the configuration in an object to further enhance the uniformity of the magnetic field in the gap space. As a typical modification in this regard, for example, the field-adjustment plates 16A and 16B each have a circular projection 161A or 161B called "a rose shim" protruded ringwise along the periphery of the flat body of the field-adjustment plate 16A or 16B, respectively.
Two combinations each consisting of the back yoke 10A or 10B, permanent magnet 14A or 14B and field-adjustment plate 16A or 16B, respectively disposed up and down symmetrically are magnetically connected with four columnar yokes 12A, 12B, 12C and 12D to form a closed magnetic circuit. Further, gradient coils 18A, 18B are each mounted on the surface of the field-adjustment plate 16A or 16B surrounded by the circular projection or first shim 161A or 161B. The height of the gradient coils 18A and 18B is such that the surface thereof facing the gap space is approximately coplanar with the outer surface of the circular projections 161A and 161B.
It is usual in the opposed-magnet magnetic circuit assembly that evaluation of the uniformity of the magnetic field in the gap space is undertaken in terms of the distribution of the magnetic field within an imaginary space at the center portion of the gap space having an spherical or ellipsoidal extension, which is referred to as the evaluation space hereinafter. When the field-adjustment plates are each a simple disk without the circular projection, the magnetic field in the equatorial portion of the evaluation space is smaller than in the polar portions thereof while, when the field-adjustment plate is provided with the circular projection as is illustrated in FIGS. 2A and 2B, the magnetic field in the equatorial portion of the evaluation space is increased due to the geometrical proximity of the equatorial portion to the surfaces of the upper and lower circular projections 161A and 161B so that the uniformity of the magnetic field throughout the evaluation space can be improved. A further proposal has been made to provide, in addition to the peripheral circular projections 161A and 161B, a plural number of circular projections having a smaller cross sectional profile than the peripheral ones on each of the bottom surfaces 162A and 162B of the field-adjustment plates 16A and 16B, respectively. As is understood from the above given description, the configuration of the field-adjustment plates is a very important factor having influences on the uniformity of the magnetic field within the evaluation space.
While it is indispensable in the opposed-magnet magnetic circuit assembly that the upper and lower back yokes 10A and 10B are connected by a plural number of connecting yokes which serve to form a closed magnetic circuit by leading the magnetic fluxes generated by the upper and lower permanent magnets 14A, 14B, the form and number of such connecting yokes can be different depending on the particular design of the magnetic circuit assembly. For example, each of the connecting iron yokes is in a columnar form and the number thereof is two or, as is illustrated in FIGS. 2A and 2B, four or in the form of a plate and the number thereof is two. Opposed-magnet magnetic circuit assemblies are mostly constructed by using four columnar connecting yokes in view of the easiness in the magnetic field adjustment and easiness in the assemblage works of the magnetic circuit assembly as is disclosed in Japanese Utility Model Publication 2-44483. The magnetic field in the evaluation space in the axial direction, i.e. in the direction of the columnar connecting yokes, is decreased as a trend because the magnetic fluxes are attracted by the columnar yokes. With an object to compensate this decrease in the magnetic field, a proposal has been made to provide a shunt yoke for shortcircuiting of magnetic fluxes in the direction of the opening so as to decrease the magnetic field in the direction of the opening.
It is a usual practice that a magnetic circuit assembly is designed by the method of numerical analysis taking into account a combination of the above described various factors so as to enable generation of a magnetic field of high uniformity within the evaluation space. It is, however, rather rare that, even if a magnetic circuit assembly could be designed so as to enable generation of a magnetic field of high uniformity, a magnetic field having uniformity of as high as designed can be obtained in an actual magnetic circuit assembly constructed according to the design. Rather, the uniformity of the magnetic field is usually subject to a great decrease as compared with the uniformity as designed as a consequence of overlapping of the unavoidable variations in the magnetic characteristics of the permanent magnets and errors in the mechanical working of the various members and assemblage of the members. Accordingly, a procedure of magnetic-field adjustment is indispensable after completion of the assemblage of the parts into a magnetic circuit assembly in order to obtain a highest possible uniformity of the magnetic field in the evaluation space.
The above mentioned magnetic field adjustment is performed in two levels of coarse adjustment by mechanical shimming and fine adjustment by magnetic-material shimming. The mechanical shimming is performed by the adjustment of the inclination of the back yokes, adjustment of the position of the field-adjustment plates, adjustment of the shunt yokes, insertion of a magnetic piece into the back yokes and so on. In the magnetic-material shimming, the uniformity of the magnetic field is improved by bonding magnetic pieces of varied volumes to appropriate spots on the field-adjustment plate or on a shim plate installed independently from the field-adjustment plates.
The magnetic materials used in the above mentioned magnetic-material shimming include soft magnetic materials such as iron and iron-based alloys, nickel and nickel-based alloys, amorphous magnetic materials and the like and hard magnetic materials such as hard ferrite magnets, rare earth-based magnets and the like as well as bonded magnets thereof. These magnetic materials are used in the form of a chip or in the form of a thin plate.
The magnetic field adjustment by shimming with a soft magnetic material encounters a difficulty when a plurality of the magnetic pieces are laid one on the other because no linear relationship is held between the number of the pieces and the amount of magnetic field adjustment. This difficulty is particularly great when the amount of magnetic field adjustment is large. This method, however, is advantageous in respect of the precision of fine adjustment because very small chips and thin plates or sheets can be readily prepared from a soft magnetic material.
On the other hand, the method of shimming with a magnet material is advantageous in respect of easiness of adjustment because a linear relationship is held between the number of the shimming members and the amount of magnetic field adjustment although the method is not suitable for precise and fine adjustment of the magnetic field because difficulties are encountered in the preparation of very small chips or thin plates of the magnetic material.
It would be an idea to employ the above described two different types of shimming materials in order to fully utilize the respective characteristics of the different materials. This approach of combined use of different types of shimming materials, however, has another problem prohibiting actual application thereof in the conventional shimming adjustment because the magnetic fluxes generated in the magnet materials influence on the soft magnetic material to destroy the linearity of the magnet. This is one of the important problems to be solved in the adjustment to establish uniformity of the magnetic field in a magnetic circuit assembly.
As is mentioned before, the opposed-magnet magnetic circuit assembly is constructed by connecting the upper and lower back yokes with four columnar connecting yokes. The magnetic circuit assembly of this type is advantageous in respect of the high efficiency with little leakage of the magnetic fluxes because the magnetic flux emitted in one of the upper and lower permanent magnets is led efficiently to the opposite pole of the other permanent magnet through the yokes.
It is therefore important in order to improve the efficiency of the magnetic circuit that the leakage of magnetic fluxes is minimized from the yokes. This requirement is of course satisfied when the yokes have a large cross sectional area sufficient to lead all of the magnetic fluxes so as not to cause magnetic saturation within the yokes although this approach is under limitations by the necessary increase in the overall weight of the magnetic circuit assembly. In addition, the opposed-magnet magnetic circuit assembly works with a high working efficiency of the magnetic circuit when the magnetic field to be obtained in the gap space is approximately in the range from 1000 Oe to 2000 Oe from the standpoint of the working point of permanent magnets. Although an opposed-magnet magnetic circuit assembly capable of generating a magnetic field of 3000 Oe or higher has been developed, such an assembly is not always very satisfactory in respect of the utilization efficiency of the permanent magnets.
A further problem in the opposed-magnet magnetic circuit assembly relative to the efficiency of the magnetic circuit is leakage of the magnetic flux from the field-adjustment plates to the yokes. This is a phenomenon that the magnetic flux, which should be led from one of the-field-adjustment plates to the other field-adjustment plate through the gap space is partly lost by leakage to the yokes resulting in a decrease in the magnetic efficiency. Although this problem can be solved at least partly by increasing the distance between the respective permanent magnets and the yokes, this means is not practical due to the necessary increase in the iron weight.
An opposed-magnet magnetic circuit assembly has a further problem, in common with the magnetic circuit assemblies of other types such as those of the superconductivity magnet-type and dipole ring magnet-type, when an MRI instrument is constructed with the magnetic circuit assembly built therein and a patient lies in the magnetic field surrounded by the magnetic circuit assembly, namely, the patient more or less receives a sense of oppression and feels unpleasantness of entering a narrow cave even if he is not claustrophobic because the space in which the patient is lying is not open on both sides of the patient. This problem of claustrophobism can be partly alleviated in the opposed-magnet magnetic circuit assembly by increasing the transverse width of the assembly and increasing the distance between the permanent magnets and the yokes though accompanied by a disadvantage due to the increase in the weight of the yokes. This problem in the dipole ringmagnet magnetic circuit assembly can be alleviated only by increasing the inner diameter of the ringwise assembly of the segment magnets while such a means is absolutely impractical because the overall weight of the segment magnets increases at a rate proportional to the square of the diameter if not to- mention the further increase of the weight of the magnets because the length of the magnets in the axial direction also must be increased in order to ensure the necessary uniformity in the magnetic field.
The encaved condition of the patient lying in an MRI instrument has another problem that accessibility of a medical engineer to the patient is necessarily restricted and visual monitoring of the patient from outside is disturbed thereby although monitoring of the patient is usually conducted by a television system in which, however, the field of vision is very limited and the contrast of the image reproduced on the screen is usually not high enough necessitating direct visual monitoring. This problem, of course, can be solved at least partly by enlarging the space in which the patient is lying while such a means cannot be undertaken without being accompanied by a great disadvantage due to the increase in the overall weight of the yokes and magnets as is discussed in Nippon Rinsho (Japan Clinics), volume 41 (1983), page 254. Accordingly, it is eagerly desired to develop a permanent-magnet magnetic circuit assembly of high magnetic efficiency by which the oppressive sense which the patient receives when he is lying in an MRI instrument can be alleviated and the accessibility of medical engineers to the patient is ensured without a substantial increase in the weights of the yokes and magnets.